Carburetor pissing gas from overflow when hot!!

So I picked up a 1979 KZ400B cafe project the other day for a decent price. The guy said he went through the carbs, engine, etc and everything looked good.

I'm pretty familiar with carburetors. I took the carbs off yesterday, no gunk in them, there is no wear and tear on the float needles, floats work fine, they aren't filling with gas and "sinking."

I pulled the tank off when I pulled the carbs. The petcock has no OFF position, or the off position isn't working? I just read about On, Reserve and Prime... interesting. I think that is what my petcock is/has. Someone needs to explain that one to me. Shouldn't I have an OFF position?

Anyways, 2 days ago I went for a decent ride, no problems got the engine nice and hot. When I got home parked it, almost immediately the overflows started pissing gas, not just a little either, and it was out of the left carb only(lower carb when kickstand is up). So I plugged the overflows and it started dripping out of the pod filter!! AGHHH.

So I start flipping the petcock switch, seeing if they would stop. Wouldn't stop. I then popped the cap off the tank to make sure there was no pressure pushing gas down into the carb, not the problem. So I pulled the tank quickly and got the gas away from the hot engine/pipes all the while thinking I'm going to burst into a ball of flames! I kept flipping the switch on the petcock and gas was still coming out of the d$*% thing. So I stuck the vacuum hose from the petcock on the end of the fuel feed nipple. Stopped that!

So after it cooled down I pulled the carbs and that's where I checked everything out. Everything looked exceptionally clean. I could tell the owner before me had gone through those things with a fine tooth comb. All passages were clear, needles great, floats were old but no leaks and looked fine. Got it back together, put the carb back on, tank back on, started it up; no leaking gas. Rode it around, didn't get it hot still no leakage. So today, I was prepared, I grabbed a little locking pair of tweezers. I went out and took a decent ride, got it hot. Got it home, shut it off and it starts pissing gas again, so I pinched the fuel line with the tweezers.

So I'm wondering, can the heat from the engine make the petcock or carburetor act differently, making it force gas into the carb? Or maybe it's the fuel and vacuum lines getting hot? The rubber for the lines is a gray rubber and it's very flexible, not the normal clear or black fuel lines I'm used to.

What the heck is wrong?!

Also:
If I was to get another petcock, what should I get? I want one with an OFF position.
Should I switch out the fuel and vacuum lines? I should put an in-line gas filter on there.
Where should I run the overflow lines off the carb? I can't find a good line.

I've also got a slight smoke coming off the lower part of the engine, right above the carbs(Tranny?) when it gets hot. Is this normal? Oil was just changed.

And should I have something on the end of the hose coming off the top of the engine that normally goes into the air box?

Anyone that wants to enlighten me, I'd be hugely appreciative.

Thank you!

Pictures

Also. What kind of tank is that off of? Stock? It doesn't look stock from the pictures of stock kz400B's I've been looking at. Sorry the last picture is upside down

The picture with my hand in it, I've got my middle finger next to the vacuum line. Is the vacuum line in the right place on the carb?

Thanks again!

Your petcock has an ON, RES, and PRI setting. Because the bike has a vacuum line connected to the petcock, gas will only flow in the on position while the engine is running. The ON position is used as the correct setting while the bike is turned off as no fuel should be flowing. You should sniff test your crankcase for gasoline. It sounds like your float heights may be set incorrectly causing fuel to overflow into your cylinders and into your crankcase. If the float heights are set too far off it can also cause fuel to leak into your air box or out of your pods. Fuel in the crankcase will also cause your bike to smoke from the bottom of the engine

So what does prime and reserve do? When the tank is off the bike, how do I get gas to quit flowing out of it?

Prime should only be used if the bike has been sitting for a longer period of time. It fills your carbs with gas when the engine is off. It doesn't require vacuum for fuel to flow. Make sure when you use the prime setting you turn the knob back to the on position or you'll come back to your bike with all the gas from your tank all over the floor. The RES position will draw fuel from the bottom of the tank and should only be used when you're low on gas. When the tank is off the bike you can set it to either ON or RES and fuel should not be flowing out of it. If fuel is flowing in the ON or RES position and the engine is not turned on, you have a problem with the petcock

Remember - NO SMOKING or other activities that could ignite the fuel while working on carbs!

Regarding the petcock -

Your petcock is the type that is operated by a vacuum line from the carbs. When your bike's engine is turned off, the lack of vacuum from the carbs allows a diaphragm inside the petcock to close off the fuel flow. You can easily test the operation of the diaphragm and its spring. With the engine off and the petcock in the run position (NOT "Prime") remove the vacuum line and the fuel line from the petcock. Sit a catch can under it in case fuel drips out. No fuel should drip out. If it does, the petcock has some internal damage and should be rebuilt or replaced.

Regarding the carb overflow -

Fuel coming out of the overflow hose (the little hose on the bottom of the carb) is the common symptom of the 3 possible problems described below.

The float valve is not sealing. This may be (and probably is) just a matter of some minor dirt in the float valve and is very simple to remedy and can be done with the carbs still on the bike. Turn off fuel; drain carb by loosening big brass screw near bottom; and remove the 4 little screws on the bottom of the carb bowl. This will enable you to remove the carb bowl. You will then see the float. If you carefully remove the float you will see a stubby little needle that mates with a brass orifice; together, these two pieces are the float valve. Assuming there is no obvious damage to the needle or seat, use a Q-tip and some carb cleaner to clean the seat of the orifice and the needle. Be careful not to bend the tang that is attached to the float. This tang is the piece that the bottom of the float valve needle rests upon, and it determines the fuel level in the bowl, so bending it will change the fuel level. With the float valve cleaned it should no longer leak. To prevent a re-occurrence a quality inline fuel filter should be installed between the fuel tank and carbs. If there was damage the needle and seat should be replaced as a set; they are available.

The brass overflow tube inside the carb bowl is damaged. When you remove the carb bowl you will see a brass tube attached inside and rising toward the top of the bowl. This is the overflow tube and it leads directly to the overflow nipple on the bottom of the carb. Normally, the fuel level within the bowl rises somewhat close to the top of this tube. If the tube becomes cracked or separates from the bottom of the carb bowl fuel will flow through it and out through the overflow hose. Checking the condition of the tube is very easy after you have removed the carb bowl from the carb. Simply hold the bowl level and fill it up near the top of the tube with water of alcohol and see if it leaks. A slight crack in the brass overflow tube can be difficult to find. You can connect a rubber tube to the overflow nipple, fill the bowl with water, put finger over the open end of the brass overflow tube and blow in the rubber end and look for bubbles. If it does the best remedy is to replace it although some folks have found creative ways to repair them.

The fuel level is set too high. This condition occurs when someone has bent the float tang as mentioned above. This condition can be checked using what is referred to as the "clear tube test." The test involves attaching one end of a clear piece of flexible tubing to the carb drain hole and holding the other end of the tube above the bowl/carb joint. When the fuel is turned on it should rise in the tube to a level 2.5 - 4.5 mm below the bowl/carb joint. If the fuel rises higher than that level it may run into the cylinders or over the upper end of the brass overflow inside the carb bowl (mentioned above) in which case it will run out the overflow hose. This condition is corrected by removing the float and gently bending the tang to raise the float valve needle position. Doing this may take several tries before achieving the desired fuel level.

Correcting each of these three conditions involves removing the carb bowl. This task can be made easier if a short screwdriver bit is used, and depending on which carb you are fixing a mirror may help you locate the screws on the bottom of the carb bowl. Also, it is not a bad idea to have a new carb bowl gasket available in case the old gasket is damaged while removing the carb bowl.

Since your bike doesn't leak fuel all the time, it is possible that dirt gets in the float valve and then is flushed out by fuel. Do you have an inline fuel filter between the tank and carbs? If not, you may want to install one. Ed

Perfect. So my petcock is broken, because when I take it off, fuel flows in all 3 positions.

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